1. Field of the Invention
The present invention relates to a variable-frequency antenna apparatus installed in a mobile telephone or the like and a radio communication apparatus.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2002-158529, Japanese Unexamined Patent Application Publication No. 2005-318336, and WO 2004/109850 disclose antenna apparatuses.
In the antenna apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2002-158529, the open end of a loop-shaped radiation electrode faces an electrode portion on the side of a feeding end with a gap therebetween, and a capacitor is formed between the open end and the electrode portion on the side of the feeding end. If a high-frequency current is supplied in the antenna apparatus, the antenna apparatus operates at a resonance frequency in a basic mode and a resonance frequency in a higher mode. By changing the gap between the open end of the radiation electrode and the electrode portion on the side of the feeding end so as to change the value of the capacitor, it is possible to change the resonance frequency in the basic mode and the resonance frequency in the higher mode.
In the antenna apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2005-318336, a parallel radiation electrode pattern is connected in parallel to a surface-mount antenna component so as to form a parallel resonance circuit. The parallel resonance circuit is disposed in a non-ground region. If a high-frequency current is supplied in the antenna apparatus, the antenna apparatus operates at a resonance frequency in a basic mode and a resonance frequency in a higher mode. By changing a gap between a pair of electrodes forming a capacitor portion of the surface-mount antenna component so as to change the value of the capacitor portion, it is possible to change the resonance frequency in the basic mode and the resonance frequency in the higher mode.
In the antenna apparatus disclosed in WO 2004/109850, a loop-shaped radiation electrode including an open end and a feeding end facing the open end with a gap therebetween is disposed in a non-ground region, and a variable-frequency circuit including a variable-capacitance element is provided on a loop path of the radiation electrode. It is possible to change a resonance frequency in a basic mode and a resonance frequency in a higher mode using the variable-frequency circuit. Furthermore, by controlling the variable-capacitance element, it is possible to make a frequency variable bandwidth wider than the bandwidth of the radiation electrode.
However, the above-described antenna apparatuses have the following problems. In the antenna apparatuses disclosed in Japanese Unexamined Patent Application Publication No. 2002-158529 and Japanese Unexamined Patent Application Publication No. 2005-318336, since the resonance frequency in the basic mode and the resonance frequency in the higher mode are changed by changing the gap between electrodes so as to change the value of the capacitor formed between these electrodes, the resonance frequency in the basic mode and the resonance frequency in the higher mode are simultaneously changed.
In the antenna apparatus disclosed in WO 2004/109850, although it is possible to perform bandwidth control over a wide frequency band using the variable-frequency circuit, as in the antenna apparatuses disclosed in Japanese Unexamined Patent Application Publication No. 2002-158529 and Japanese Unexamined Patent Application Publication No. 2005-318336, the resonance frequencies in the basic mode and the resonance frequency in the higher mode are simultaneously changed and cannot be separately changed.
In a monopole antenna such as the antenna apparatus disclosed in WO 2004/109850, a current I1 in the basic mode and a current I2 in the higher mode (a harmonic having a frequency of three times that of the basic mode) are distributed as illustrated in FIG. 18. Accordingly, by providing a variable-frequency circuit 200 provided with a variable-capacitance element at a position corresponding to zero of the current I2 in the higher mode as indicated by a broken line, it is possible to change the resonance frequency in the basic mode and fix the resonance frequency in the higher mode. That is, only the resonance frequency in the basic mode can be changed. However, if the variable-frequency circuit 200 is provided at the position corresponding to zero of the current I2 in the higher mode, the variable-frequency circuit 200 is provided at a position corresponding to a current I1′ in the basic mode. The current I1′ is smaller than a current Imax of the feeding portion. Accordingly, even if the value of the variable-capacitance element is changed, a bandwidth in which the resonance frequency in the basic mode is variable becomes narrow. The antenna apparatus therefore lacks in practicability.